The only drawback I have with FF nowadays is history management.
I have enabled 'infinite history' (do not delete old history, ever) so I can keep a journal of what I've visited when. The history, as large as it might turn out, is just a few MB of an sqlite3 database (places.sql) -- problematic is the management of it using the Firefox UI. Searching is laggy and deletion of swaths of entries is impossible as it makes the history manager UI hang for many minutes or even hours (=essentially I always kill Firefox when I do this by mistake). I suspect the GUI constructs a view of the sqlite db using single GUI objects tied to single DB entries and therefore has maximum overhead.
Editing the places.sql file directly via the sqlite3 CLI (with Firefox shut down) is a matter of (milli)seconds at best.
If I had the resources to compile FF in reasonable time I would give developing the patch a shot myself, but browser development is not an option with my current hardware, and I do not have a build server set up.
PS. The fact that Chrome does not support tagged bookmarks is another nail for its coffin. Makes it impossible to organize 10000s of bookmarks, and search on them.